1. Field of the Invention
This invention relates to field emission cathodes and method for making. More particularly, group III nitride thin films grown on silicon or other substrates to form a high density of field emission micro-tips and method for making are provided.
2. Description of Related Art
Field emission of electrons from solid surfaces can provide a cold cathode for use in displays and other devices of vacuum microelectronics. In field emission, flow of electrons from the surface of a solid material into a surrounding vacuum occurs under the influence of an applied electric field. In order to be emitted, an electron must propagate through a potential barrier between the surface and the vacuum. Quantum mechanical tunneling makes such propagation possible.
The potential barrier that the electron must overcome depends on the material's "electron affinity," which is a constant for each given surface and is different for different materials. Most materials have large positive electron affinity, but a few materials exhibit low or even negative electron affinity. These latter materials need a very low applied electric field for field emission to occur. Examples of materials with low or negative electron affinity are specific surfaces of diamond, gallium nitride, aluminum nitride, boron nitride and other group III nitrides. Alloys of Group III nitrides are also included among low or negative electron affinity materials--ternary alloys such as AlGaN, InGaN, InAIN and quaternary alloys such as AlGaInN. These materials have composition-dependent bandgap and, therefore, composition dependent electron affinity.
Prior art field emission devices rely on various phenomena for their properties. In one approach, the enhancement of field emission current and lowering of the threshold voltage is achieved by making an emitter in the shape of a sharp tip. Such a geometry concentrates the applied voltage to a very small area and creates strong electric fields. These microtip cathodes have been used for years in field emission displays. Such microtips are described, for example, in patents to Spindt, including U.S. Pat. No. 4,857,799. Experience reveals several limitations of the microtip cathodes. They are difficult to manufacture, variation of emission current can be significant over large areas such as those required for flat displays, the lifetime of the emission tip is low due to the large current flow through a very small area and the high vacuum required during operation can be expensive.
In another approach to develop field emitters, materials with reduced electron affinity are used to form a planar surface. An example of this approach is provided in U.S. Pat. No. 5,686,791, suggesting the use of amorphic diamond with imbedded micro-crystallites that presumably have low or negative electron affinity. Diamond surfaces are very sensitive to oxygen, however, and must be packaged under a vacuum, which increases fabrication cost. This limits the use of diamond for such applications as ion source cathodes or micro electromechanical systems (MEMS), for example, where oxygen is often a process environment. Also, integration with well-developed Si technology is very difficult in the case of diamond.
Nitrides have also been suggested as planar field emitters. For example, the paper by Sowers et al, ("Thin films of aluminum nitride and aluminum gallium nitride for cold cathode applications,." (App. Phys Lett. 71 (16), Oct. 20, 1997)) discusses nitride layers deposited on a silicon carbide substrate. Gallium nitride is a wide bandgap material with a low electron affinity (2.7 eV) that has recently attracted attention as a material for field emission (FE) devices--because of this low electron affinity and its high chemical and mechanical stability. Field emission cathodes fabricated from GaN should have longer lifetimes because of their high sputtering resistance and low sensitivity to residual gases, especially oxygen. Only a few reports have been published regarding the field emission properties of GaN. Recently, Underwood et al ("GaN field emitter array diode with integrated anode") and Kozawa et al ("Fabrication of GaN field emitter arrays by selective area growth technique") reported field emission from GaN micro-sized hexagonal pyramids grown by selective-area metal organic chemical vapor deposition on sapphire substrates, . In both studies the threshold electric field for emission was high: 195 V/.mu.m and 100 V/.mu.m, respectively, in comparison with the best results for diamond--about 0.5-2.0 V/.mu.m. In addition, emission current densities were low. Most results to date are on insulating sapphire substrates, which require significant processing to achieve the sharp structures necessary to enhance surface electron emission. Silicon carbide is a less commonly used conductive substrate for GaN hetero growth. Both types of substrates are relatively expensive compared with silicon, and are not available in large sizes such as the 10-inch wafers of silicon. Size limitations can be a significant limitation in applications such as displays.
There have been several reports of cubic and hexagonal GaN deposition on Si wafers. (Yang et al, "High quality GaN-InGaN heterostructures grown on (111) silicon substrates") Although the GaN/Si materials exhibited good optical and electrical properties, field emission data from these films is unavailable.
What is needed is a stable field emission device that can reduce fabrication cost of field emitters, making them compatible with current Si growth technologies, and make, for the first time, monolithic integration of field emitters with Si-based driving circuitry possible. The field emitter should be chemically stable in oxygen so that cathodes can operate at much higher pressures and oxygen can be tolerated, and it should be long-lived even at high current levels.
Uniform emission current over large areas is also desired.